A conventional electric power steering system includes a torque sensor as exemplarily disclosed in JP 2005-43071A. The torque sensor is connected to an input shaft (steering shaft) and an output shaft (intermediate shaft) relatively rotatably through a torsion bar. The torque sensor includes an input shaft rotation angle sensor and an output shaft rotation angle sensor, which detect rotation angles of the input shaft and the output shaft, respectively. The torque sensor calculates a twist angle of the torsion bar as a difference between the input shaft rotation angle and the output shaft rotation angle, and applies the calculated twist angle of the torsion bar to a motor control unit. The motor control unit calculates a required steering assisting power based on the twist angle and drives the motor in a direction to reduce the twist of the torsion bar.
The conventional electric power steering system thus generates appropriate steering assisting power when the torsion bar is twisted by a steering wheel operated by a vehicle driver. The torsion bar is twisted by torque applied from not only an input shaft side but also an output shaft side. The torque applied from the input shaft side is generated by a steering wheel when the driver operates the steering wheel. The torque applied from the output shaft side is generated by road surface reactive force.
In the electric power steering system, only the twist angle calculated from the input shaft rotation angle and the output shaft rotation angle is applied to the motor control unit. Therefore it is not possible to check whether the torsion bar is twisted from the input shaft side (that is, torque application from the steering wheel side) or the output shaft side (that is, torque application from tire wheel side).
As a result, even when the torsion bar is twisted by toque applied from the output shaft side, the motor control unit drives the motor in a direction to reduce the amount of twist of the torsion bar. The transfer of reactive force from the road surface is thus limited, and hence the driver hardly senses the road surface condition based on the vibrating motion of the steering wheel.